change SPI index for BeagleBone/PocketBeagle #106

Change SPI port index because SPI0 is /dev/spidev1.x and SPI1 is /dev/spidev2.x on both:
* the current recommended BeagleBoard.org Debian image: Debian 9.5 (Stretch) from 2018-10-07
* the newest Debian 9.8 (Strecth) snapshot from 2019-04-07

This change is to fix issue #106: inconsistent spidev index on AM335x-based boards like BeagleBone Black and PocketBeagle.